1. Field of the Invention
The present invention relates to a slow-wave structure for monolithic microwave integrated circuits.
2. Description of Related Art
Various kinds of microwave circuits, such as filters and couplers, require a transmission line having a length on the order of a wavelength at an operating frequency. This is particularly true for providing wideband and high-Q components that have a phase shift that is a linear function of frequency. Conventional transmission lines, however, have wavelengths that are too long compared to the desired physical dimensions of integrated circuits. In other cases, such as optical modulators, the electrical wavelength of the conventional transmission line is too long compared to the modulation wavelength of an optical, acoustic or other kind of wave to be coupled to the electromagnetic wave propagating on the transmission line.
In order to shorten the electrical wavelength of a transmission line, it is known to provide a periodic variation in the inductance or capacitance of the line so as to form a "slow-wave" structure. The slow-wave structure functions like a transmission line so long as the variations are spaced at a distance much smaller than electrical wavelength. The period variation increases the inductance or capacitance per unit length of the line and thereby decreases the phase velocity of the electromagnetic wave, which is inversely proportional to the square root of the product of the inductance and capacitance per unit length. It is also known that the periodic variations in inductance and capacitance can be chosen to select the impedance of the slow-wave structure, which is proportional to the square root of the ratio of the inductance per unit length to the capacitance per unit length.
A slow-wave structure for monolithic GaAs microwave integrated circuits is disclosed in Bastida U.S. Pat. No. 4,340,873 and his article "Periodic Slow-Wave, Low-Loss Structures For Monolithic GaAs Microwave Integrated Circuits," Electronics Letters, 13th September 1979, Vol. 15, No. 19, pp. 581-582. The slow-wave structures include stripline waveguides periodically shunted by high-Q overlap capacitors. In FIGS. 1 to 5, the Bastida patent shows a substrate of semi-insulating semiconductor material (GaAs) upon which are disposed two parallel conducting bands connected together by transverse strips spaced apart at distances less than the signal wavelength. A strip of dielectric material such as silicon dioxide is disposed on top of and perpendicular to the transverse strips. Finally, a conducting strip is disposed on top of the insulating material. As shown in FIG. 2 of the Electronics Letters article, the conducting strip 5 itself can be a periodic structure to enhance the slow-wave effect.
Another slow-wave structure for monolithic GaAs microwave integrated circuits is disclosed in Seki & Hasegawa, "Cross-tie Slow-wave Coplanar Wageguide on Semi-insulating GaAs Substrates," Electronic Letters, 10th December 1981, Vol. 17, No. 25, pp. 940-941. Seki & Hasegawa propose a coplanar waveguide slow-wave structure built on a semi-insulating GaAs substrate and having spaced apart crossties made of conducting material. The crossties are formed directly on the semi-insulating GaAs substrate. Then a dielectric layer is disposed on the crossties. Finally, the coplanar waveguide is disposed on the dielectric layer.
Slow-wave propagation also occurs along a microstrip transmission line disposed over and forming a Schottky junction with a semiconductor medium. The slow-wave effect occurs due to different partitioning of the magnetic and electric field energy between the Schottky depletion layer and the semiconductor layer. As disclosed in Neidert, et al., U.S. Pat. No. 4,630,011, a variable phase shift can be obtained by biasing the Schottky junction so as to vary the thickness of the depletion layer.